The invention relates to photodetectors, and more particularly relates to photodetectors of the photomultiplier tube type. In its most immediate sense, the invention relates to photomultiplier tubes of the type which are suitable for use in the detectors of gamma cameras and PET (Positron Emission Tomography) scanners such as are used in nuclear medicine.
A photomultiplier tube is a device which converts incident light radiation to an electrical signal. In a photomultiplier tube, light is made incident upon a photocathode, which produces photoelectrons in response. The photoelectrons are usually directed to a series of dynodes made e.g. of metal alloys whose surface oxides have high secondary emission ratios. (Examples of such alloys are AgMg, CuBe and NiAl, whose surface oxides are, respectively, MgO, BeO and Al.sub.2 O.sub.3.) When a photoelectron strikes a dynode, the impact causes a plurality (typically, from 3 to 30) secondary electrodes to be emitted, which produce yet more secondary electrons when they strike the next dynode in the series. At the end of the series, the original photoelectrons have been greatly multiplied in number, and when the final electron beam strikes the anode of the photomultiplier tube it produces an output signal which is large enough for use in subsequent electronic circuitry, even if the originally-incident light is of very low intensity.
Photomultiplier tubes are used in large numbers in scintillation cameras (e.g. gamma cameras and PET scanners). In these devices, radiation is made incident upon a scintillation crystal, where the radiation interacts with the crystal to create a flash of scintillation light (a "scintillation event"). The light from this scintillation event is viewed by photomultiplier tubes, which produce electrical output signals that are used in subsequent circuitry and data processing apparatus.
In both classes of devices, photomultiplier performance is critical. Furthermore, in both classes of devices, spatial resolution (generally considered the key measure of performance) depends upon the size of the photomultiplier tubes; the smaller the tubes, the better the resolution. (This is because a conventional photomultiplier tube only indicates whether light from a scintillation event is present at its input surface and not where that event is located. To determine the locations of scintillation events with sufficient precision, i.e. with a spatial resolution which is better than the dimensions of the photocathode of the photomultiplier tube, it is necessary to use a plurality of photomultiplier tubes and to cause the scintillation light to spread out over an area which is sufficiently large to encompass more than one photomultiplier tube at a time. In conventional detector configurations, photomultiplier tubes having diameters of 75 mm can produce spatial resolution of 4 mm; photomultiplier tubes having diameters of 50 mm can produce spatial resolutions on the order of 3 mm.) However, because the expense of photomultiplier tubes is substantial and generally does not vary with tube size, and because each photomultiplier tube requires electrical circuitry to e.g. provide power, control gain, etc., designers of gamma cameras and PET scanners are constrained to trade off performance versus cost and to accept larger photomultiplier tubes and degraded spatial resolution as necessary consequences of manufacturing an affordable instrument.
It has recently been proposed to use semiconductor diodes or avalanche photodiodes ("APD") instead of the dynodes and anode which are conventionally employed in photomultiplier tubes; the resulting combination device has been referred to as a Vacuum Avalanche Photodiode or a Hybrid Photomultiplier Tube ("VAPD" or "HPMT"). This produces a position-sensitive device, improves the linearity of the device and also improves the signal-to-noise ratio at the output.
Where an APD is so employed, the resulting VAPD or HPMT has a potential capability to produce substantial cost savings in the detectors of gamm cameras and PET scanners. This is because it would be possible to use fewer VAPDs or HPMTs and to thereby reduce per-tube associated costs.
However, for such a resulting VAPD or HPMT to be technically feasible in a gamma camera, the light image at the photocathode must be accurately minified at the APD. This would not be so if, as is conventional, the resulting VAPD or HPMT uses a glass envelope with a flat input end and a conventional photocathode.
This is because a conventional photocathode structure, mounted to the flat input end of a glass envelope, applies the same potential to all points on the input end. This would distort the response of the tube if the tube were to be position-sensitive; in a position-sensitive device, changes of location of input light will not produce appropriately corresponding changes of location at the anode (APD). As a result, the resulting VAPD or HPMT would produce distorted output signals.
Furthermore, such a VAPD or HPMT would also be unsuitable for PET scanner applications. A PET scanner works by detecting pairs of annihilation quanta which are simultaneously emitted from a common annihilation site. When such "coincidence detection" techniques are utilized, the system "looks" for two quanta which occur within tens of nanoseconds, and perhaps even within nanoseconds, of each other. It is therefore important that the time response of the VAPD or HPMT be independent of the location of the scintillation event (e.g. at the center of the input end of the photomultiplier tube or at the edge thereof).
This would not be so in VAPDs or HPMTs with flat input ends. This is because photoelectrons from the center of the photocathode would arrive earlier (would have a shorter "transit time") at the avalanche photodiode than would photoelectrons from the edge of the photocathode, and the size of the flat input end would would make the discrepancy in timing unacceptably large.
It would therefore be advantageous to provide a photomultiplier with a flat input end and an APD which would accurately minify onto the APD the light image at the input end and which would make the photoelectron transit time independant of location on the photocathode.
One object of the invention is to provide a photomultiplier with a flat input end and an APD, which would cause light images at the input end to be accurately minified on the APD and would also provide for a transit time which was independant of location on the photocathode.
Another object is, in general, to improve on known devices of this general type.
The invention proceeds from the known proposition that an ideal shape for a photocathode in a photomultiplier tube is a section of a sphere. In accordance with the invention, a model is constructed of the potential distribution in a photomultiplier tube with a spherical-type input end, i.e. in a photomultiplier tube in which the input end is a section of a sphere. Furthermore, a model is constructed to determine what this potential distribution would be, as measured in a transverse plane immediately adjacent the input end. Then, a photocathode structure is configured to produce the thus-determined potential distribution. This is done by applying, to the input end, metallized regions which produce the desired potential distribution when connected to appropriate sources of electrical potential. A photocathode is then applied to the interior of the photomultiplier such as to be electrically connected to the metallized regions. When such a photocathode structure is affixed at a flat input end of the glass envelope of a photomultiplier tube, the photomultiplier tube acts like a photomultiplier tube with a spherical-type input end.
Advantageously, the glass envelope has flat sides and is shaped to be rectangular (further advantageously, square) in cross-section. This permits the photomultipliers to be densely packed together. In this embodiment, conductors are mounted to the sides and produce on them a potential distribution characteristic of a photomultiplier which is cylindrical in cross-section, as measured at flat surfaces having the same shape as the envelope.
Further advantageously, the photocathode is deposited using conventional techniques onto metallized regions located inside the envelope. Electrodes, such as Kovar pins, are placed in the envelope; the interior ends of the electrodes make contact with the metallized regions and the exterior ends of the electrodes are available for connection to suitable electrical potentials.